Outcomes of Advanced Gastroesophageal Cancer Patients with Equivocal HER2 Expression with or without ERBB2 Gene Amplification

Background: Prior studies have shown that patients whose tumor overexpresses Her2 at 3+ level by immunohistochemistry (IHC) fare better than those whose tumor overexpresses Her2 at 2+ level (with ERBB2 amplified). Therefore, it would be important to compare the outcome of patients whose tumor expresses Her2 at 2+ level but further classify by gene amplification studies as positive or negative. Methods: We retrospectively identified patients with advanced gastroesophageal adenocarcinoma with low Her2 protein expression (2+ by IHC) whose tumors were evaluated for gene amplification of ERBB2 by fluorescence in situ hybridization (FISH). All patients received first-line therapy, and trastuzu-mab was added according to Her2 status. We compared overall survival (OS), progression-free survival (PFS), and overall response rate (ORR) of the entire cohort and compared Her2-positive tumor patients' outcomes with Her2-negative tumor patients' outcomes. All patients had treatment response assessments and follow-ups at our institution. Results: We identified 87 patients whose tumors expressed Her2 at 2+ level. 51 (58.6%) were Her2-negative and 36 (41.4%) were Her2-positive by FISH. For the entire cohort, the median OS was 26 months (95% confidence interval 16.6-37.6), and the median PFS was 12.2 months (95% confidence interval 9.7-19.3). Median OS, median PFS, and ORR did not differ between Her2-positive and Her2-negative patients (p = 0.70, p = 0.60, p = 0.91, respectively). Conclusions: Our data suggest that patients with Her2 positivity or negativity when tumors have lower Her2 protein expression (2 + by IHC) have similar clinical outcomes. Further research is warranted in this cohort.